Burning or gasifying coal or petcoke produces a gas containing various contaminants. In coal-fired power stations, flue gas desulfurization (FGD) is often employed to scrub most of the sulfur dioxide from flue gas. Similarly, a gas scrubbing step is utilized in most gasification processes used for coal and petroleum coke to produce syngas. In wet scrubbing processes, an alkaline agent dissolved in water, reacts with and removes contaminants from the flue gas. Wet scrubbers typically require continuous blowdown to limit the accumulation of corrosive salts and suspended solids washed from the gas stream.
Most wet scrubbers typically spray the flue gas with a slurry containing limestone (CaCO3). Sulfur dioxide (SO2) in the flue gas reacts with the limestone to form calcium sulfite (CaSO3). This product is further oxidized to produce gypsum (CaSO4.2H2O) by introducing air into the scrubber. Although the compositions of scrubber waste streams vary depending on the fuel type and scrubbing reagent used, the waste streams are generally a chloride solution saturated with gypsum and contain calcium, magnesium, sodium, potassium and trace amounts of heavy metals. These salts are extremely soluble and have high boiling point elevations.
Contaminants found in a flue gas waste stream from a flue gas scrubber using limestone to remove contaminants are given below in Table 1.
TABLE 1TypicalContaminantConcentrationsCalcium4250mg/lMagnesium950mg/lSodium590mg/lPotassium25mg/lIron15mg/lChloride10,000mg/lSulfate1320mg/lNitrate90mg/lFluoride12mg/lSilica28mg/lTotal Suspended Solids5,000mg/lm-Alkalinity280mg/l as CaCO3
Contaminants found in a flue gas waste stream from a flue gas scrubber using dolomitic limestone to remove contaminants are given in Table 2.
TABLE 2TypicalContaminantConcentrationsCalcium1,200mg/lMagnesium9,900mg/lSodium1,700mg/lPotassium50mg/lZinc4.6mg/lChloride9,000mg/lSulfate28,000mg/lBromide20mg/lTotal Suspended Solids7,000mg/l
Often, these waste streams cannot be discharged into the environment without chemical treatment. Traditionally, wastewater from a limestone scrubbing process requires at least two precipitation/flocculation stages due to the wide variation in the optimum pH values for the precipitation of the metals present. In addition, the presence of selenium, nitrates, and organics in the purge stream often require biological treatment prior to discharge. Such treatment methods may reduce the suspended solids, metals, acidity and oxygen demand, but do not reduce the chloride or total dissolved solids.
Moreover, as discharge limits become more stringent, physical, chemical and biological treatment methods may not reduce concentrations to the levels required for discharge of some chemical species. When conventional treatment methods are unable to treat scrubber waste streams to produce an effluent that meets the requirements of a discharge permit, an evaporation process may be employed. Evaporation processes evaporate substantially all the water from the waste stream and remove substantially all dissolved solids from the waste stream, resulting in a zero liquid discharge into the environment.
There are generally two approaches that have been used to achieve zero-liquid discharge of blowdown from wet scrubbers using evaporation. The first approach includes an initial clarification process to reduce suspended solids in the influent water stream. Clarified water flows to an evaporator tank where it is neutralized with acid before the water stream is preheated in plate heat exchangers. Preheated water is then deaerated using steam from the evaporator. Typically, most of the water is evaporated in a falling film evaporator at atmospheric pressure. A rotary or spray dryer is used to remove the remaining water and produce a dry solid which is sent to a landfill.
The second approach involves chemically softening the scrubber blowdown after an initial clarification process. Generally, lime and soda ash are added to the scrubber blowdown to cause substantially all of the magnesium and calcium ions to precipitate as magnesium hydroxide and calcium carbonate respectively. The precipitate is settled and removed from the scrubber blowdown. Typically, the softened scrubber blowdown is neutralized with acid before the water stream is preheated and deaerated. The scrubber blowdown is then sent to a falling film evaporator or multiple effect evaporator to vaporize the water and concentrate the scrubber blowdown. The concentrated blowdown is further evaporated in a forced circulation evaporator. A slurry of precipitated salts formed in the forced circulation evaporator is sent to a centrifuge or pressure filter to separate the solids from the water.
When evaporating solutions containing high solubility salts such as calcium, magnesium or ammonium chloride at atmospheric pressure, the temperature increases as the concentration increases. Thus, many waste stream evaporation systems operate at very high temperatures. However, many salts hydrolyze at high temperatures leading to corrosion of the evaporation system. Thus, many evaporators require the use noble alloys to withstand corrosion.
Conventional evaporation-crystallization processes for waste streams containing salts have numerous drawbacks and shortcomings. As discussed above, many processes require clarification of the feed stream prior to evaporation. In addition, conventional processes use chemicals to soften or otherwise condition the feed prior to evaporation. Clarification processes require clarifiers and the use of chemicals which increase the footprint of the treatment system as well as the capital costs and overall maintenance. Therefore, there is a need for an evaporation-crystallization process that is economical, efficient, and which is highly effective in removing contaminants from waste streams without the use of clarification or chemical conditioning.